1. Field of the Invention
The present invention relates to a process for the preparation of macrocyclic polyethers, commonly referred to as "crown ethers". The invention relates in particular to the preparation of 1,4,7,10,13,16-hexaoxacyclooctadecane, also known as 18-crown-6.
2. Description of the Prior Art
Synthetic macrocyclic polyethers are of great interest in that they contain intra-molecular cavities which are fit to accomodate alkali metal or alkaline-earth metal ions depending on the particular shape of the macrocyclic polyether applied. Therefore, salts of alkali metals and alkaline-earth metal ions, which are substantially insoluble, become solubilized in the presence of certain macrocyclic polyethers. It is also possible to dissolve such salts in organic solvents such as benzene or toluene in the presence of certain macrocyclic polyethers. Macrocyclic polyethers, and especially 18-crown-6, have found recognition as solvents for chemical reactions as well as phase-transfer catalysts for chemical reactions.
A number of synthetic techniques has been proposed for preparing macrocyclic polyethers. Among those, reference may be made to the catalytic oligomerization of ethylene oxide as described in U.S. Pat. No. 3,928,386 leading to mixtures of various macrocyclic polyethers. The reaction of tetraethyleneglycol with bis(2-chloroethyl) ether in the presence of potassium hydroxide and tetrahydrofuran without addition of water is described in "Synthesis" (1976) pages 515-516. However, even after reaction time of at least 18 hours, moderate yields are obtained and the working-up procedure includes a distillation or distillative decomposition of the macrocyclic polyether (complex) produced. It is further known from J. Chem. Soc. Chem. Comm., 1978, pages 504-505 that macrocyclic polyethers may be prepared by reacting (un)substituted polyethylene glycols having suitable ethylenoxy units with a sulphoxyl chloride in the presence of an alkali metal hydroxide in an aprotic solvent such as dioxane or dimethoxyethane, followed by pyrolysis of the salt complex obtained in order to liberate the macrocyclic polyether.
It is remarkable, however, that despite a fast growing interest in macrocyclic polyethers large scale preparations have not yet been reported.